Multimedia services in wireless communication systems are in increasing demand. Developments are being made to achieve data transmissions with higher capacity and higher speed. Therefore, efficient use of limited radio frequency resources is of increasing importance. To this end, a multiple-input multiple-output (MIMO) system using a multi-antenna has been employed. The multi-antenna system uses two or four transmitting antennas to send packets.
FIGS. 1(A) and 1(B) depict mathematical representations (e.g., matrices) of an exemplary signal (e.g., packet) transmitted through two and four transmitting antennas, respectively. FIGS. 2(A) and 2(B) depict mathematical representations of an exemplary re-transmission packet transmitted through each antenna when initial transmission of the packet fails.
Referring to FIGS. 2(A) and 2(B), if an error occurs in packet transmission, the transmission system alters the packets to enable them to be transmitted through each antenna. The transmission then re-transmits the packets. Such a re-transmission method may allow SNR (signal to noise ratio) gain at a receiving side to be increased by making an initially transmitted packet and a re-transmitted packet have a STTD (space time transmit diversity) structure.
However, the conventional packet re-transmission using two or four transmitting antennas may have problems, such as waste of radio resources.
Furthermore, in the field of data transmissions, an ARQ (Automatic Repeat reQuest) method is a type of error data re-transmission method. ARQ refers to a response message that indicates whether or not a receiving station has properly received data after transmission. The ARQ response method comprises three types: a Stop-and-wait ARQ, a Go-back-N ARQ and a Selective-repeat ARQ.
FIG. 3(A) illustrates three types of ARQ methods.
Referring to FIG. 3(A), the Stop-and-wait ARQ is a method where a transmitting station transmits data and waits to receive an ACK (acknowledgement) or NACK (non-acknowledgement) message (signal) from a receiving station. Then, the transmitting station transmits new data or re-transmits the previously transmitted data. The Go-back-N ARQ is a method where data is continuously transmitted regardless of receiving a response. When a NACK signal is received, data is re-transmitted in turn starting from the data indicated by the NACK signal. The Selective-repeat ARQ is a method where data is continuously transmitted, and only the data for which a NACK signal has been received is re-transmitted.
For packet data transmissions, in order to prevent errors that may be generated in a high speed transmission environment employing a high data rate (e.g., 2 Mbps, 10 Mbps or higher), a suitable coding rate or modulation method (e.g., Rc=5/6, 3/4; Mod=16-QAM, 64QAM) has been applied to communication systems. In addition to this, an ARQ method suitable for the high-speed transmission environment, namely, a Hybrid ARQ (HARQ) method has been proposed.
In the ARQ method, when an error is generated, the corresponding information is discarded, whereas in the HARQ method, information with an error is stored in a buffer and combined with re-transmitted information and FEC (Forward Error Correction) is applied thereto. Thus, the HARQ method employs the ARQ method with FEC additionally performed (HARQ=FEC+ARQ).
The HARQ method may be divided into four types, as described below.
FIG. 3(B) shows a Type I HARQ method, by which an error detection code is added to data in order to preferentially detect an FEC. If the data (packet) still includes an error, the transmitting station is requested to re-transmit the data (packet). The packet with an error is discarded and the re-transmitted packet uses the same FEC code as that of the discarded packet.
FIG. 3(C) shows a Type II HARQ method, which is also called an IR (Incremental Redundancy) ARQ. Referring to FIG. 3, according to the Type II HARQ method, a first (initially) transmitted packet is not discarded but is stored in a buffer and then combined with re-transmitted redundancy bits. Upon re-transmission, only the parity bits (excluding the data bits) are re-transmitted. The parity bits that are re-transmitted are different for each re-transmission.
FIG. 3(D) shows a Type III HARQ method, which is a special case of the Type II HARQ method. Here, each packet is self-decodable. Re-transmissions are performed for each packet that includes data as well as portions having errors. Compared with the Type II HARQ method, the Type III HARQ method may achieve more accurate decoding, but has less coding gain.
FIG. 3(E) shows a fourth method called ‘Type I with soft combining method’, which combines the function of the Type I HARQ method plus a function of storing the data first (initially) received from a receiving station and combining such with re-transmitted data. The method is also referred to as ‘metric combining’ or ‘chase combining.’ The method is advantageous with respect to an Signal to Interference Plus Noise Ratio (SINR). Furthermore, the same parity bits for the re-transmitted data are used.
The MIMO system will now be described as follows. The MIMO system is a wireless system in which a terminal and a base station transmit and receive signals using one or more antennas and diversity gain may be obtained on the time axis or on the frequency axis. The MIMO system employs two types of methods: STTD (Space-Time Transmit Diversity) and Collaborative Spatial Multiplexing (SM). STTD is a method for obtaining diversity gain through use of antennas and time axis information by transmitting two or more signals via two or more antennas, while Collaborative SM is a method for allocating two or more terminals to a single radio resource.
For example, when the base station has two antennas, a MIMO matrix ‘A’ of equation (1) shown below may be used to transmit signals S1 and S2 according to the STTD method. Thus, equation (1) shows a MIMO matrix of the STTD method for 2-antenna transmission.
                    A        =                  {                                                                      S                  1                                                                              -                                      S                    2                    *                                                                                                                        S                  2                                                                              S                  1                  *                                                              }                                    (        1        )            
In equation (1), the rows of the matrix represent the signals sequentially transmitted through the first and second antennas, while the columns of the matrix refer to a time sequence. In other words, on a first channel, the first antenna is used to transmit the signal S1 and the second antenna is used to transmit the signal S2, while on the second channel, the first antenna is used to transmit the signal −S2* and the second antenna is used to transmit the signal S2. Assuming that reception values received at the receiving end over time are r1 and r2, then r1 and r2 may be calculated by equation (2) shown below. Thus, equation (2) represents the reception signals for 2-antenna transmission:r1=h1·S1+h2·S2 r2=h1·(−S2*)+h2·S1*  (2)
In equation (2), h1 and h2 represent a channel state (condition or status) of the first and second antennas, respectively. In addition, when the base station has two antennas, in order to transmit the signals S1 and S2 according to the Collaborative SM method, a MIMO matrix B, such as in equation (3) shown below may be used. Thus, equation (3) shows a MIMO matrix of the Collaborative SM method:
                    B        =                  {                                                                      S                  1                                                                                                      S                  2                                                              }                                    (        3        )            
In a communications system using three or four transmission antennas and performing re-transmissions, when a first spatial multiplexing transmission is performed, signals represented by the vectors shown in [Table 1] and [Table 2] are transmitted and each element of each vector is transmitted via each antenna. [Table 1] shows an example of a HARQ re-transmission vector when using three antennas, while [Table 2] shows an example of a HARQ re-transmission vector when using four antennas. Note: The tables are located on pages 6 and 7.
In this embodiment, when re-transmission is required, the odd numbered re-transmissions and the even numbered re-transmissions are discriminated when performing re-transmissions. For an odd number re-transmission, a re-transmission “option” (i.e., a type of space-time code incremental redundancy for a matrix) may be selected such that one of several re-transmission vectors is selectively used for the re-transmission.
For the downlink, the information for selecting an option may be indicated by varying a codeword of a NACK signal that is received. For example, in case of ACK, the codeword “0,0,0” may be sent, while in case of NACK, “4, 7, 2” may be sent to indicate Option 1, while “1, 2, 3” may be sent to indicate Option 2, and “3, 6, 5” may be sent to indicate Option 3. In this manner, ACK and NACK may be distinguished, and in case of NACK, the particular option to be used (Option 1, 2, or 3) may be distinguished.
However, for the uplink, because the related art ACK/NACK signal is expressed as a single bit, such option selection may not be indicated by using the related art ACK/NACK signal.
TABLE 1Odd number re-Even number re-Initial transmissiontransmissiontransmissionSpace time code incremental redundancy for matrix      S    2          (      0      )        =      [                                        S                          i              +              1                                                                        S                          s              +              2                                                                        S                          i              +              3                                            ]        S    2          (      odd      )        =      [                                        -                          S                              i                +                2                            *                                                                        S                          i              +              1                        *                                                            S                          i              +              3                        *                                ]   (Option 1)      S    2          (      even      )        =      [                                        S                          i              +              1                                                                        S                          s              +              2                                                                        S                          i              +              3                                            ]         S    2          (      odd      )        =      [                                        -                          S                              i                +                3                            *                                                                        S                          i              +              2                        *                                                            S                          i              +              1                        *                                ]   (Option 2)       S    2          (      odd      )        =      [                                        -                          S                              i                +                1                            *                                                                        S                          i              +              3                        *                                                            S                          i              +              2                        *                                ]   (Option 3)
TABLE 2Odd number re-Even number re-Initial transmissiontransmissiontransmissionSpace time code incremental redundancy for matrix      S    2          (      0      )        =      [                                        S                          i              +              1                                                                        S                          i              +              2                                                                                                                        S                                      i                    +                    3                                                                                                                        S                                      i                    +                    4                                                                                            ]        S    2          (      odd      )        =      [                                        -                          S                              i                +                2                            *                                                                                                                        S                                      i                    +                    1                                    *                                                                                                      -                                      S                                          i                      +                      4                                        *                                                                                                                        S                          i              +              3                        *                                ]   (Option 1)      S    2          (      even      )        =      [                                        S                          i              +              1                                                                        S                          i              +              2                                                                                                                        S                                      i                    +                    3                                                                                                                        S                                      i                    +                    4                                                                                            ]         S    2          (      odd      )        =      [                                        -                          S                              i                +                3                            *                                                                                                                        -                                      S                                          i                      +                      4                                        *                                                                                                                        S                                      i                    +                    1                                    *                                                                                                      S                          i              +              2                        *                                ]   (Option 2)       S    2          (      odd      )        =      [                                        -                          S                              i                +                4                            *                                                                                                                        -                                      S                                          i                      +                      3                                        *                                                                                                                        S                                      i                    +                    2                                    *                                                                                                      S                          i              +              1                        *                                ]   (Option 3)
As stated above, the related art MIMO system has at least the following problems. When re-transmission is performed, the field indicating the ‘nth’ transmission is included in the Information Element (IE), but when one of several options of the NACL signal is selected to be used in sending a re-transmission vector, there is no definition or procedure that specifies whether the base station or the terminal should perform the selection and then send the re-transmission vector, and there is no definition or procedure as to how such selection should be made.